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A report describes a non-standard
direct digital synthesizer (DDS) implementation
that can be used as part of a
coherent transponder so as to allow any
rational turnaround ratio to be exactly
achieved and maintained while the
received frequency varies. (A coherent
transponder is a receiver-transmitter in
which the transmitted carrier is locked
to a pre-determined multiple of the
received carrier’s frequency and phase.
That multiple is called the turnaround
ratio.)

The report also describes a general
model for coherent transponders that
are partly digital. A partially digital
transponder is one in which analog signal
processing is used to convert the signals
between high frequencies at which
they are radiated and relatively low frequencies
at which they are converted to
or from digital form, with most of the
complex processing performed digitally.
There is a variety of possible architectures
for such a transponder, and different
ones can be selected by choosing different
parameter values in the general
model.

Such a transponder uses a DDS to create
a low-frequency quasi-sinusoidal signal
that tracks the received carrier’s
phase, and another DDS to generate an
IF or near-baseband version of the transmitted
carrier. With conventional DDS
implementations, a given turnaround
ratio can be achieved only approximately,
and the error varies slightly as the
received frequency changes. The nonconventional
implementation employed
here allows any rational turnaround
ratio to be exactly maintained.

This work was done by Larry R. D’Addario
of Caltech for NASA’s Jet Propulsion
Laboratory. NPO-47460

This Brief includes a Technical Support Package (TSP).

Achieving Exact and Constant Turnaround Ratio in a DDS-Based Coherent Transponder (reference NPO-47460) is currently available for download from the TSP library.

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